Blood cell development represents an orderly sequence of steps by which stem cell populations differentiate into mature erythroid, myeloid, megakaryocytic, and lymphoid cell types. It has become increasingly evident in the last ten years that many cell fate decisions during blood development and expansion are governed by a group of secreted molecules known as cytokines or hematopoietic growth factors. A prototypical cytokine is erythropoietin (Epo), which is the major growth factor required for red blood cell development. Like most hematopoietic cytokines Epo exerts survival (anti-apoptotic), mitogenic, and specific differentiative effects on target cells. Epo largely appears to be specific for cells of the erythroid lineage, thus, Epo, its cell surface receptor (EpoR), and cellular signaling apparatus represents an excellent prototype for investigation because of its crucial role in hematopoietic physiology, its widespread clinical, its restricted biologic functions, and the existence of mouse models of leukemia involving activation of the EpoR. However, further work is needed to clarify the precise stages at which irreversible red cell commitment decisions are made, the molecular and cellular mechanisms that mediate these decisions, the factors that trigger commitment in vivo, and by extension how these processes are affected by, or can give rise to, leukemic transformation. To determine the contribution of specific cytoplasmic EpoR tyrosine residues to the complete biologic response to Epo in endogenous red cells, we will express tyrosine mutated EpoR isoforms in developing erythroid progenitors, in vivo. To study the contribution of cellular context in determining the outcome of Epo/EpoR engagement, chimeric receptor technology will be used to express functional domains of nonEpoR cytokine receptors in the erythroid compartment, in vivo, and evaluate the connection to red cell development, and leukemic transformation. To determine if sole activation of the cytosolic janus kinase, JAK2, will support red cell development we will express chimeric EpoR/JAK2 proteins in developing red cells, in vivo, and evaluate the connection to red cell development, and leukemic transformation.